Edge of the Orison, by Iain Sinclair

The essential conceit of Edge of the Orison is that Iain Sinclair pursues "the traces of the mad poet John Clare", while investigating his wife's claim to be Clare's distant relation. He is not writing a biography, and thus never "finds" him in that conventional sense. Where most would head for the Public Record Office, he undertakes a series of walks, shadowing, haphazardly, Clare's "journey out of Essex" in 1841, with a series of collaborators. The aim is to follow Clare through a species of divination, so that Sinclair writes as the detective of a cosmic conspiracy.

As he perceives it, the trail is laid by invisible precursors: "I re-walk, compulsively, routes other men established". If the past and present are at times indistinguishable, a foreordained reality emerges out of chaos, pointing to ultimate revelation in the near future.

The "energy lines of England" run out from Bunhill Fields cemetery, resting-place of Defoe, Blake and Bunyan; Sinclair's tour of Northampton "laid down a pattern that activated a series of overlapping narratives". Northampton, a key location, turns out to be surprisingly potent, described by Alan Moore as "a black hole from which only 'mangled information' can escape".

Intersecting lives also play their part - the fact that Tennyson and Clare were, for a while, inmates of the same madhouse; that the Ripper suspect, poet and convicted lunatic J K Stephen was a cousin to Virginia Woolf; that Wordsworth and Clare witnessed Queen Victoria's visit to Northampton, though without meeting; that Sinclair turns out to be distantly related to Samuel Beckett.

Throughout, Sinclair fights through a dense fog of unknowing - the detritus of modern life, of which he is our foremost laureate. There is the " black question mark of pubic hair" in a tablet of hotel soap; a slab of microwaved meat, "ice-pink in the middle, burnt on the outside"; and, in a Chinese restaurant, the "dense flatus of incense, aftershave, sweet gravy, cola wind and tobacco".

This is a feast, a riddle, a slowly-unravelling conundrum of the senses. And at its core is the most deranged of Romantic poets, doomed to outlive the contemporaries with whom he had once mixed (Lamb, Hazlitt, De Quincey), bearing witness, between bouts of depression and insanity, to the Victorian era. Far from "getting inside his head", the supposed objective of the biographer, Sinclair is a trustworthy recipient of posthumous energy-fields - the body-warmth surviving in Clare's snuffbox collection, for instance. When asked by the librarian at the Bodleian to determine which of two watches belonged to Shelley, he chooses correctly: "I made a pass over the meticulously positioned object. I felt the heat on my open palm."

In the wrong hands, this could be mere dandyism. Whatever strange humour he finds in those around him, Sinclair remains in earnest throughout, the most painstaking of scholars, tabulating the platonic almanack by which his life is intertwined with that of his quarry. Edge of the Orison is not best regarded as literary biography, and Sinclair himself describes it as "non-academic" (he may protest too much: it is furnished with bibliography, archival illustrations, and an admirably thorough index). Instead, regard it as the guide-book to a distinctive vision. It is, in short, a kind of love-letter to British Romanticism, an attempt to journey to its sources, though not one that ever declares itself as such.

Duncan Wu is professor of English at Oxford University